• Olof Johansson's avatar
    block: uninitialized ioc->nr_tasks triggers WARN_ON · 4638a83e
    Olof Johansson authored
    Hi,
    
    I'm using the old-fashioned 'dump' backup tool, and I noticed that it spews the
    below warning as of 3.5-rc1 and later (3.4 is fine):
    
    [   10.886893] ------------[ cut here ]------------
    [   10.886904] WARNING: at include/linux/iocontext.h:140 copy_process+0x1488/0x1560()
    [   10.886905] Hardware name: Bochs
    [   10.886906] Modules linked in:
    [   10.886908] Pid: 2430, comm: dump Not tainted 3.5.0-rc7+ #27
    [   10.886908] Call Trace:
    [   10.886911]  [<ffffffff8107ce8a>] warn_slowpath_common+0x7a/0xb0
    [   10.886912]  [<ffffffff8107ced5>] warn_slowpath_null+0x15/0x20
    [   10.886913]  [<ffffffff8107c088>] copy_process+0x1488/0x1560
    [   10.886914]  [<ffffffff8107c244>] do_fork+0xb4/0x340
    [   10.886918]  [<ffffffff8108effa>] ? recalc_sigpending+0x1a/0x50
    [   10.886919]  [<ffffffff8108f6b2>] ? __set_task_blocked+0x32/0x80
    [   10.886920]  [<ffffffff81091afa>] ? __set_current_blocked+0x3a/0x60
    [   10.886923]  [<ffffffff81051db3>] sys_clone+0x23/0x30
    [   10.886925]  [<ffffffff8179bd73>] stub_clone+0x13/0x20
    [   10.886927]  [<ffffffff8179baa2>] ? system_call_fastpath+0x16/0x1b
    [   10.886928] ---[ end trace 32a14af7ee6a590b ]---
    
    Reproducing is easy, I can hit it on a KVM system with a very basic
    config (x86_64 make defconfig + enable the drivers needed). To hit it,
    just install dump (on debian/ubuntu, not sure what the package might be
    called on Fedora), and:
    
    dump -o -f /tmp/foo /
    
    You'll see the warning in dmesg once it forks off the I/O process and
    starts dumping filesystem contents.
    
    I bisected it down to the following commit:
    
    commit f6e8d01b
    Author: Tejun Heo <tj@kernel.org>
    Date:   Mon Mar 5 13:15:26 2012 -0800
    
        block: add io_context->active_ref
    
        Currently ioc->nr_tasks is used to decide two things - whether an ioc
        is done issuing IOs and whether it's shared by multiple tasks.  This
        patch separate out the first into ioc->active_ref, which is acquired
        and released using {get|put}_io_context_active() respectively.
    
        This will be used to associate bio's with a given task.  This patch
        doesn't introduce any visible behavior change.
    Signed-off-by: default avatarTejun Heo <tj@kernel.org>
        Cc: Vivek Goyal <vgoyal@redhat.com>
    Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
    
    It seems like the init of ioc->nr_tasks was removed in that patch,
    so it starts out at 0 instead of 1.
    
    Tejun, is the right thing here to add back the init, or should something else
    be done?
    
    The below patch removes the warning, but I haven't done any more extensive
    testing on it.
    Signed-off-by: default avatarOlof Johansson <olof@lixom.net>
    Acked-by: default avatarTejun Heo <tj@kernel.org>
    Cc: stable@kernel.org
    Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
    4638a83e
blk-ioc.c 10.2 KB